Embodiments of the invention relate generally to x-ray tubes and, more particularly, to an anti-fretting coating for an attachment joint and a method of making same.
Computed tomography X-ray imaging systems typically include an x-ray tube, a detector, and a gantry assembly to support the x-ray tube and the detector. In operation, an imaging table, on which an object is positioned, is located between the x-ray tube and the detector. The x-ray tube typically emits radiation, such as x-rays, toward the object. The radiation typically passes through the object on the imaging table and impinges on the detector. As radiation passes through the object, internal structures of the object cause spatial variances in the radiation received at the detector. The detector converts the received radiation to electrical signals and then transmits data received, and the system translates the radiation variances into an image, which may be used to evaluate the internal structure of the object. One skilled in the art will recognize that the object may include, but is not limited to, a patient in a medical imaging procedure and an inanimate object as in, for instance, a package in an x-ray scanner or computed tomography (CT) package scanner.
A typical x-ray tube includes a cathode that provides a focused high energy electron beam that is accelerated across a cathode-to-anode vacuum gap and produces x-rays upon impact with an active material or target provided. Because of the high temperatures generated when the electron beam strikes the target, typically the target assembly is rotated at high rotational speed for purposes of spreading the heat flux over a larger extended area.
As such, the x-ray tube also includes a rotating system that rotates the target for the purpose of distributing the heat generated at a focal spot on the target. The rotating subsystem is typically rotated by an induction motor having a cylindrical rotor built into an axle that supports a disc-shaped target and an iron stator structure with copper windings that surrounds an elongated neck of the x-ray tube. The rotor of the rotating subsystem assembly is driven by the stator.
The target is attached to a support shaft, which is in turn supported by roller bearings that are typically hard mounted to a base plate. Thus, the target provides a thermal path to the roller bearings that can cause the roller bearings to operate at elevated temperature, compromising the life thereof. In order to minimize or reduce the operating temperature of the bearings, often a thermally resistive material is placed between the target and the bearings. The thermally resistive material, referred to sometimes as a thermal barrier, can be designed having a high thermal resistance to include using a material having a relatively low thermal conductivity, a very thin wall and additional length—all resulting in an increased thermal resistance between the target and the bearing. Thermal resistance can be further increased by introducing a bolted joint between the shaft and the roller bearings, as it is well known that contact resistance in, for instance, a bolted joint can cause a large thermal resistance and temperature drop thereacross in conduction heat transfer. As known in the art, bolted joint strength may be enhanced by designing components such that they have an interference fit, and in some instances bolts may be foregone entirely, leaving joint strength entirely to the interference fit at an interface therebetween. Not only may such designs be intended to increase thermal conductivity, bolted and/or interference joints may be introduced into a design to facilitate assembly of components (such as an anode or target assembly) during fabrication of an x-ray tube.
However, because the target is typically rotated about its axis at a high rate of speed, typically 100 Hz or more, and because the x-ray tube itself is rotated at a high rate of speed on a gantry, typically 2 Hz or more, enormous periodic loads can be generated at interfaces that join the target and other rotating components. So, high-frequency periodic loads are applied to the joint due to the target rotation and some unavoidable residual unbalance of the rotating components and low-frequency periodic loads due to the tube rotation on the CT gantry. Such loads in a bolted joint can cause bending of the joints components causing small relative motion to occur, which can cause fretting, leading to particulate generation within the x-ray tube. Fretting and particulate generation can occur in bolted joints and at interfaces that include, for instance, interference joints. In fact, particles can be generated at any interface where materials are such as in a bolted joint or an interference fit pressed together (but not fused or otherwise bonded together, such as in a welded or brazed joint, as examples). And, the effect can increase significantly with increased gantry and/or increased target rotating speed, leading to increased fretting and particulate generation as x-ray tubes are rotated faster on gantries and as targets are rotated faster within x-ray tubes.
As known in the art, particulate in an x-ray tube can degrade performance and life in a number of ways that include, for instance, accelerated bearing wear if the wear particles fall into the bearing and electrical discharge activity in the high voltage environment of the x-ray tube. Both of these issues reduce the useful life of the x-ray tube.
Accordingly, it would be advantageous to have an x-ray tube that could be rotated at a high speed on a gantry and at a high target rotational speed without a reduction in life due to particulate generation at connection joints in the x-ray tube.